Field Emission Devices
Field emission devices are devices that capitalize on the movement of electrons. A typical field emission device includes at least a cathode, emitter tips, and an anode spaced from the cathode. A voltage is applied between the cathode and the anode causing electrons to be emitted from the emitter tips. The electrons travel in the direction from the cathode to the anode. These devices can be used in a variety of applications including, but not limited to, microwave vacuum tube devices, power amplifiers, ion guns, high energy accelerators, free electron lasers, and electron microscopes, and in particular, flat panel displays. Flat panel displays can be used as replacements for conventional cathode ray tubes. Thus, they have applications in television and computer monitors.
Conventional emitter tips are made of metal, such as molybdenum, or a semiconductor such as silicon. The problem with metal emitter tips is that the control voltage required for emission is relatively high, e.g., around 100 V. Moreover, these emitter tips lack uniformity resulting in non-uniform current density between pixels.
More recently, carbon materials, have been used as emitter tips. Diamond has negative or low electron affinity on its hydrogen-terminated surfaces. Diamond tips, however, have a tendency for graphitization at increased emission currents, especially at currents about thirty mA/cm2. Carbon nanotubes, also known as carbon fibrils, have been the latest advancement in emitter tip technology. Although much work has been done in the area of carbon nanotubes as emitter tips in field emitting technologies, substantial improvement is still needed in at least three areas. These are reducing the working voltage (specific to the particular application), reducing the “turn-on” voltage, increasing emission current density, and increasing the number of emission sites. Reducing the “turn-on” voltage (and the working voltage) tends to increase the ease of electron emission and increase the longevity of the emitter tips. Increasing both the emission current and the number of emission sites increases the brightness. An increased number of emission sites will likely result in a more homogeneous emission across a given area or volume.
Carbon Nanotubes
Carbon nanotubes (CNTs) are vermicular carbon deposits having diameters of less than about one micron. They exist in a variety of forms, and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces, by high temperature carbon arc processes, where solid carbon is used as the carbon feed stock, and by simultaneous laser vaporization of graphite rods and a transition metal. Tennent, U.S. Pat. No. 4,663,230, succeeded in growing small diameter nanotubes having cylindrical ordered graphite cores and an ordered “as grown” graphitic surface uncontaminated with pyrolytic carbon. Tennent, describes carbon nanotubes that are free of a continuous thermal carbon overcoat and have multiple graphitic outer layers that are substantially parallel to the fibril axis. As such they may be characterized as having their c-axes, the axes which are perpendicular to the tangents of the curved layers of graphite, substantially perpendicular to their cylindrical axes. They generally have diameters no greater than 0.1 micron and length to diameter ratios of at least five. Such nanotubes having graphitic layers that are substantially parallel to the fibril axis and diameters between 3.5 and 75 nanometers, are described in Tennent et al., U.S. Pat. No. 5,165,909 and Tennent et al, U.S. Pat. No. 5,171,560, both of which are herein incorporated by reference.
The graphitic planes may also be oriented at an angle to the fibril axis. Such structures are often called “fishbone” fibrils or nanotubes because of the appearance of the two dimensional projection of the planes. Such morphologies and methods for their production are discussed in U.S. Pat. No. 4,855,091 to Geus, herein incorporated by reference. Fishbone fibrils are typically 10 to 500 nm in diameter, preferably from 50 to 200 nm and have aspect ratios between 10 and 1000.
Macroscopic assemblages and composites consisting of multiwall nanotubes have been described in Tennent et al, U.S. Pat. No. 5,691,054, herein incorporated by reference. Such assemblages and composites are composed of randomly oriented carbon fibrils having relatively uniform physical properties in at least two dimensions. Such macroscopic assemblages are differentiated from “as-made” aggregates by the ability to form them at any desired size. Preferably such aggregates have at least one dimension greater than 1 mm and preferably greater than 1 cm. Such assemblages may take the form of a two dimensionally isentropic mat or felt.
The carbon nanotubes disclosed in U.S. Pat. Nos. 4,663,230, 5,165,909, and 5,171,560, may have diameters that range from about 3.5 nm to 70 nm and lengths greater than 100 times the diameters, an outer region of multiple essentially continuous layers of ordered carbon atoms and a distinct inner core region. Furthermore, these multiwall nanotubes are substantially free of pyrolytically deposited carbon. All of these references are herein incorporated by reference.
As disclosed in U.S. Pat. No. 5,110,693 and references therein (all of which are herein incorporated by reference), two or more individual carbon fibrils may form microscopic aggregates of entangled fibrils. Simply for illustrative purposes, one type of microscopic aggregate (“cotton candy or CC”) resembles a spindle or rod of entangled fibers with a diameter that may range from 5 nm to 20 nm with a length that may range from 0.1 μm to 1000 μm. Again for illustrative purposes, another type of microscopic aggregate of fibrils (“birds nest, or BN”) can be roughly spherical with a diameter that may range from 0.1 μm to 1000 μm. Larger aggregates of each type (CC and/or BN) or mixtures of each can be formed.
Carbon nanotubes having a single wall comprising a single graphene sheet have been produced. These single wall carbon nanotubes have been described in Bethune et al., U.S. Pat. No. 5,424,054; Guo, et al., Chem. Physics Lett., 243:1-12 (1995); Thess, et al, Science, 273:483-487 (1996); Journet et al., Nature 388 (1997) 756; Vigolo, et al., Science 290 (2000) 1331. They are also described in U.S. patent application Ser. No. 08/687,665, entitled “Ropes of Single-Walled Carbon Nanotubes” herein incorporated by reference. Single wall nanotubes may be prepared by a variety of procedures. These may use a solid phase carbon source which is vaporized by an arc or by a laser. Alternatively, and preferably, single wall nanotubes are made catalytically from gas phase carbon precursors. There are two broad methods of such catalytic synthesis: so-called aerosol or floating catalyst processes using a gas phase catalyst precursor which is decomposed to catalytic species in the reaction zone and processes using a classical supported catalyst. Aerosol processes may advantageously employ elevated pressures of up to 100 atm. Supported catalyst processes operate at ambient pressures and may even be operated at vacuum. Preferred gas phase carbon sources are CO, CH4, ethanol and benzene. Preferred temperatures are between 500 and 1000° C.
Additional methods of producing single wall nanotubes production have been described in PCT Application No. PCT/US99/25702 and PCT Application No. PCT US98/16071 herein incorporated by reference. Single wall nanotubes are useful in a variety of applications. The tubular structure imparts superior strength, low weight, stability, flexibility, thermal conductivity, large surface area and a host of electronic properties. They can be used as reinforcements in fiber reinforced composite structures or hybrid composite structures, i.e., composites containing reinforcements such as continuous fibers in addition to single wall nanotubes. The carbon nanotubes may be treated in their as-made form or may be deposited as a film on a suitable substrate and then treated. All of these references are herein incorporated by reference. Nanotube Deposition Methodology—disclosed in Electrophoretic Deposition of Nanotubes (from U.S. Patent App'n Pub. 2003/0090190, herein incorporated by reference).
The Electrophoresis Bath
The electrophoretic deposition of the carbon nanotubes may be conducted in an electrophoresis bath. The bath consists of a chamber to contain the solution of carbon nanotubes and means for immersing two opposing electrodes separated by some distance with the carbon nanotubes between the opposing electrodes. A DC power supply, external to the bath, is used to apply a voltage between the two electrodes immersed in the bath. The cathode lead is connected to the patterned aluminum substrate and the anode lead is connected to the other electrode. Tantalum was used for the second metal. The voltage applied to the two electrodes can be adjusted to a suitable level or the voltage can be adjusted to obtain a suitable current between the two electrodes. The attachment of carbon nanotubes to the aluminum can be enhanced by a binder. The binders can be a mixture of Ag paste, carbon nanotubes and ethanol. Or the binders can be a conductive carbon paste, a conductive metal paste or a carbonizable polymer.
Electrophoretic Deposition of Carbon Nanotubes on the Substrate
A field emitter substrate is loaded into the electrophoresis bath. A plurality of cathodes are arranged on a glass substrate, and a dielectric film is formed with holes over the cathodes. Metal gates with openings which are located over the holes of the dielectric film are formed to expose the surface of the cathodes. Then, the carbon nanotubes are uniformly deposited onto the obtained substrate, on the surface of the cathodes exposed through the holes by electrophoretic deposition at room temperature.
Post Deposition Heat Treatment
After the deposition of carbon nanotube particles by electrophoresis, low-temperature heating is performed to sustain the deposition of the carbon nanotubes on the cathodes and ensure easy removal of impurities which are incorporated into the field emitter during the deposition.
Preparation of Nanotube Film on Aluminum Substrate (Example from U.S. Patent App'n Pub. 2003/0090190, herein Incorporated by Reference)
With reference to FIG. 17, a solution is formed that contains 150 ml i-propyl alcohol (IPA) and 0.44 grams of acid washed carbon nanotubes. This solution is placed in an electrophoresis bath 5000.
A patterned, aluminum coated glass substrate 5002 serves as one electrode in electrophoresis bath 5000. The pattern forms the pixel size. The smallest feature size can be ca. 1 micron. The aluminum coated glass 5002 is about 55 mm.×45 mm.×1 mm in its dimensions. The aluminum pattern size is about 9 mm×9 mm. The other electrode, tantalum (Ta) electrode 5004 is also inserted into the electrophoresis bath 5000. A spacer 5006 separates the aluminum coated glass 5002 from the tantalum electrode 5004. A DC voltage, for example between 40 to 120 volts, e.g., 100 volts is applied to the electrodes. A current between 1.0 to 5 mA, e.g., 3.8 mA, is observed between the electrodes. The duration of the preparation time can be between about 30 to about 90 minutes, e.g., 60 minutes.
FIG. 18 illustrates an alternative electrophoretic method of creating the film according to the method disclosed in UK patent application 2,353,138 described below. First, a carbon nanotube suspension is created. The carbon nanotube particles can have lengths from about 0.1 to about 1000 microns. The suspension can also include a surfactant, e.g. an anionic, ionic, amphoteric or nonionic, or other surfactant known in the art. Examples of suitable surfactants include octoxynol, bis(1-ethylhexyl)sodium sulfosuccinate, and nitrates of Mg(OH)2, Al(OH)3 and La(OH)3.
The suspension is then subjected to an electric field to charge the carbon nanotube particles. The intensity of the electric field and the time for which the electric field is applied define the thickness of the carbon nanotube layer. Greater intensity and longer time yield thicker layers.
With reference to FIG. 18, the field emitter substrate 6030 is loaded into the electrophoresis bath 6000 containing a carbon nanotube suspension 6010. An electrode plate 6020 is also installed in the electrophoresis bath 6000 spaced apart from the field emitter substrate 6030. The cathode of a DC power supply 6040, which is installed outside of the electrophoresis bath 6000, is connected to the other cathodes of the field emitter substrate 6030 and the anode of the DC power supply 6040 is connected to the electrode plate 6020. Then, a bias voltage of about 1 to-about 1000 volts is applied from the DC power supply 6040 between the electrode plate 6020 and the cathodes of the field emitter substrate 6030.
As a positive voltage of the DC power supply 6040 is applied to the electrode plate 6020, carbon nanotube particles charged by positive ions in the carbon nanotube suspension 6010 migrate to and are attached to the exposed cathodes of the field emitter substrate 6030, which results in the formation of a carbon nanotube film in the pattern of the exposed cathodes.
The height of the printed carbon nanotube film, also known as the ink, coating, or paste, may be less than 10 microns and the space which isolates carbon nanotube cathodes from the indium tin oxide anode with indium tin oxide and phosphor is about 125 microns.
The electrophoresis process can be applied to both diodes and triodes. For applications to a diode, an electric field having opposite charges to those on the surfaces of the carbon nanotube particles is applied to exposed electrode surface of a field emitter substrate for selective deposition of carbon nanotube particles thereon. For application to a triode having gates, a weak positive electric field is applied to the gates while a positive electric field is applied to the electrodes of the field emitter substrate, which avoids deposition of carbon nanotube particles on the gates. In particular, the electrode plate is connected to the anode of the DC power supply and the cathodes of the field emitter substrate are connected to the cathode of the DC power supply. As a positive potential is applied to the gates, the gates repel positive ions in the carbon nanotube suspension at the surface, while the exposed cathodes of the field emitter substrate, which are connected to the cathode of the DC power supply, pull positive ions of the suspension through the holes. As a result, the carbon nanotubes are deposited only on the entire exposed surface of the cathodes, not on the gates of the field emitter substrate. At this time, carbon nanotube particles are attracted to the field emitter substrate and are oriented substantially horizontal, or substantially parallel to the substrate, which allows the carbon-nanotube particles to smoothly migrate through the holes to the cathodes, and thus the carbon nanotubes can be deposited.
The film can also be prepared similarly to the carbon ink disclosed in European Patent Application EP 1 020 888 A1—Carbon ink, electron-emitting element, method for manufacturing and electron-emitting element and image display device.